Chirality, derived from the Greek term, represents a fundamental symmetry-breaking phenomenon in nature [1]. It is defined as the property of an object in three-dimensional space that cannot be superimposed with its mirror image through translation or rotation operations, such as the human left and right hands, which are mirror images of each other but cannot be perfectly overlapped [2,3]. Chirality widely exists in physics. Circularly polarized wave exhibits typical chiral characteristics, divided into left-handed circularly polarized waves (LCP) and right-handed circularly polarized waves (RCP) [4,5]. The electric field vectors of LCP and RCP rotate counterclockwise and clockwise, respectively, and they demonstrate significantly different optical properties when interacting with chiral materials, such as circular dichroism (CD) and optical rotation [6,7]. These properties hold significant application in fields like optical sensing[[8], [9], [10]], optical polarizers [11,12], and chiral recognition of biomolecules [[13], [14], [15], [16]]. Therefore, in-depth study of chirality is crucial for unraveling the essence of life phenomena and developing safe and effective drugs [17].

Although numerous chiral materials exist in nature, they generally suffer from two distinct drawbacks: weak chiral responses excited by natural materials and difficulties in achieving flexible regulation [18]. In recent years, chiral metamaterials with complex structures have been reported to achieve significant chiral effects (e.g., chiral responses under Oblique incidence conditions) [19,20]. However, for terahertz chiral metasurfaces, many application scenarios require a narrower line width, simple structural design, vertical incidence, stronger circular dichroism (CD), and a higher Q factor to achieve an enhanced chiral optical response[[21], [22], [23]].

The integration of bound states in the continuum (BIC) into metasurfaces represents a prominent strategy to enhance light-matter interactions and achieve high Q factor devices [24,25]. BIC are exotic wave phenomena where eigenmodes remain confined within the radiative continuum, manifesting as non-radiative states with theoretically infinite Q factors [26,27]. In practical implementations, these BIC are typically engineered into quasi-BIC with finite Q factor through two principal mechanisms: symmetry-protected BIC realized via structural perturbations in C2-symmetric metasurfaces and accidental BIC achieved through parameter tuning under specific interference phase conditions (Friedrich-Wintgen or Fabry–Pérot resonance criteria) [[28], [29], [30], [31], [32], [33]]. However, existing BIC-based chiral metasurfaces are constrained by two critical limitations: They generally support only single-band chiral responses, failing to meet the demand for multi-frequency applications, and their Q factors often exhibit significant degradation at peak chiral response frequencies, restricting their utility in precision sensing and lasing applications. Recent studies have demonstrated chiral response characteristics with Q factors [[34], [35], [36]]. For example, in 2024, a research team reported a structure exhibiting a circular dichroism (CD) value of 0.99 and a Q factor exceeding 46,000 [37]. However, this work demonstrates distinct advantages: the proposed all-dielectric structure enables simultaneous multi-band chiral responses at 1.027 THz and 1.172 THz while maintaining Q factors above 104, achieves near-unity CD values (0.986 and 0.995) without significant Q factor degradation, and employs a simpler, passive design compared to active tunable alternatives. In contrast to active tunable designs, this study adopts a simpler all-dielectric configuration. Unlike single-band quasi-BIC chiral metasurfaces [38] where Q factors degrade at maximum CD, the proposed structure maintains high Q factors even at peak CD performance.

In this study, based on the principle of symmetry breaking, we designed a BIC metasurface that can achieve multiple chiral resonances in the terahertz (THz) band. The metasurface unit cell comprises two semicylindrical silicon pillars with C2 symmetry broken by offsetting and downsizing one pillar. Numerical simulations and theoretical analysis reveal that this configuration simultaneously excites multiple ultra-high-Q chiral responses, achieving circular dichroism (CD) magnitudes approaching unity under identical structural parameters. Two of these resonances correspond to quasi-BIC states. Furthermore, resonance frequencies and linewidths can be systematically tuned via geometric parameter adjustments. Multipole decomposition and electromagnetic field distribution analysis at quasi-BIC conditions confirm the underlying physical mechanisms. These results demonstrate the potential of BIC chiral metasurfaces for applications requiring narrow-linewidth, high-Q responses, such as THz chiral sensing and circularly polarized laser sources.